Do not run pip install if no packages to install

This MR fixes #74 (closed).

It also fixes a temporary issue with Sphinx-4.0.1, which does not properly pin Jinja and MarkupSafe dependencies, creating uninstallable environments.

conda/meta.yaml

@@ -41,7 +41,7 @@ requirements:
     - gitpython
     - python-gitlab
     - requests
-    - sphinx
+    - sphinx!=4.0.1
     - pyyaml >=5.1
     - twine <3
     - lxml